Site Objective
This website aims to provide a knowledge base for 2nd generation Mazda MX6 & 626, Ford Probe and all other cars using various displacements of the Mazda V6 engine in both Otto & Miller Cycles. Some areas are also applicable to the 1.8 & 2.0 I4 engines also utilised in these and other vehicles.
Coverage includes general maintenance, problem & workaround areas, plus developments of both engine and cars for objectives of comfort, handling & safety, as well as acting as a general resource for technical information. Updates of this site are essentially when I can find a few minutes available.
The V6 configuration appears in 1.8, 2.0, 2.25 and 2.5 litre displacements. The 2.25V6 engine is of particular interest as it is the worlds only Miller-Cycle as opposed to Otto-Cycle engine. The MC employs a twin-intercooled Autorotor supercharger to deliver 210-230lb/ft at 3500rpm yet exceptional fuel-economy, translating into on the road performance from Xedos9-MC/Millenia-S cars of 0-60mph in just 7.9secs with the 4-speed automatic gearboxes. The supercharger by virtue of the compact Lysholm-Screw design, the worlds most efficient, allows it to be sited deep in the 60-degree-V of the V6 and with the twin intercoolers sitting above.
The 2.5V6 appears in KL & KL-ZE forms, the latter being a 200bhp sports derivative of the KL achieving a remarkable 80bhp/litre without the expense of variable valve timing. All normally aspirated V6s feature a Variable Resonance Intake System (VRIS), the same concept as later used by Porsche on their 3.6 and Ferrari in much more sophisticated form on their V12. VRIS was first used by Mazda in their older 148bhp 2.0i-16 UK engine in 1989 and ensures the widest possible area under the torque curve rather than a purely headline peak figure. The VRIS contributes to the V6s noted low fuel consumption, with the 7500rpm or 7800rpm limits rarely being called upon, and averages of 27-29mpg (imperial) in urban driving and 32-36mpg (imperial) elsewhere being the norm.
Engine Architecture
V6 engines are an all-alloy DOHC 24V 60-degree V6 configuration, with fully split-crankcase (as 911 flat-6) with four 4-bolt mains and a further pair of bolts at each bearing section. The crankshaft is forged, exhaust valves are stainless steel. The split-crankcase affords exceptional rigidity over traditional bearing cap solutions and contributes not only to high-rpm capability, but also reduces NVH data to set a benchmark in 1992 on introduction. Vibration is mitigated further by reducing reciprocating mass of the rods & pistons as much as possible, and for longevity the short-stroke design subjects piston & ring to velocities lower than those for the I4 engines. Quantitatively, the 2.5V6 Mean Piston Speed (MPS) is a very low 3170ft/min at redline and the 2.0I4 MPS is 3928ft/min at its redline, with the V6s very short stroke producing low crank angles thus minimising main & rod bearing load. Regarding ring design, the 2.5V6 uses 1.5mm top ring allowing a Maximum Piston Acceleration (MPA) of 77,000ft/sec^2, with the engine at redline experiencing an MPA of 51,354ft/sec^2. The 2.0I4 uses a 1.17mm top ring allowing a Maximum Piston Acceleration (MPA) of 105,000ft/sec^2, with the engine at redline experienceing an MPA of 70,157ft/sec^2. Thus both engine designs have exceptional design margins, the V6 in particular is considerably under-stressed.
In terms of caveats, the engine architecture presents few for both V6 & I4 engines, the former being over-engineered even in boosted applications, the latter being a derivative of the 1.8 185/235bhp AWD B-series application. The valvetrain is the wear issue if oil is not regularly changed and HLAs are allowed to remain noisy for 10,000s of miles. In designing the valvetrain Mazda had a difficult balance to achieve: hydraulic lash adjusters (HLAs) small enough to allow a 7500-7800rpm redline without succumbing to valve-float or resorting to solid tappets as Honda have to, yet HLAs large enough to resist collapse & dirty oil. Whilst the V6 with very minor changes is capable of over continuous 8900rpm from Ford testing in the US, dirty oil is known to clog the small oil-control ball valve of HLAs creating a characteristic tap-tap at idle if left untreated.
Initial HLA noise during cold engine startup is normal if only for a short period. During startup the 24 HLAs are in varying degrees of compression depending on where the 4 cams stopped at shutdown. Those HLAs experiencing the greatest compression from the cam lobe (against a 175lb/in spring) will take longer to pump up at a cold-start compared to their less compressed counterparts, compounded by the time between starts. HLA pump-up time is also to a lesser extent dependent on where the HLAs are on the oil path, with HLAs at the top left of the rear bank (cyl #1) taking slightly longer. The use of centre-oiled cams (as favoured by Porsche in race engines over spray-bar systems) helps to ensure fast targeted oiling in both cold start & high-rpm conditions. The actual mechanism of the noise from HLAs is a function of the degree of lash between the cam lobe and HLA surface: when HLAs are pumped up with clean oil & operating correctly they maintain this lash at zero for all rpm (and thus achieving the self-adjusting maintenance free operation of HLAs).
Translating design into realised longevity appears to have been effective. In the UK two 626s over 3 years covered 247,000 & 300,000 miles from 2.0I4 & 2.5V6 respectively. Only failures on the V6 were (Mitsubishi) alternator at over 200,000 miles, and (steretypical for pre 5th Jan 95 cars) failure of rear callipers (Sumitomo, Mitsubishi; ex gratia UK) and distributor (Ignitor Module, Mitsubishi, failure due to poor choice & implementation of power transistor, discounted via Roebuck Mazda USA or Ford in Canada).
In race applications, the 2.0V6 was used by Ford in the Mondeo/Countour (the Duratec lacks a forged crankshaft), and by Mazda in the Supertouring, the 2.5V6s in MX6s racing in several countries successfully. In race form with the addition of twin turbochargers outputs of 328bhp @6500rpm & 318lb/ft @4000rpm, and 553bhp @8000rpm & 451lb/ft @3750rpm have been proven. A twin turbo 2.0V6 Xedos6 achieved 589bhp. A Vortech V5 supercharged application netting 230bhp & 260lb/ft is more commercially available. Other kits under development range from forged pistons & rods to match the stock forged crankshaft at under 1500$US (Probe Industries), to 2.7 litre upgrades.
Mazda 626/MX6 & Probe Problem Overview
93-94 V6s
- – Minor problems are largely localised to 1993-94 years, but despite this are resolved economically. For example, the Ignitor Module is available separately where stereotypical symptom of ignitor failure is engine cutting-out with tachometer instantly dropping to zero (due to loss of ignitor signal) & only restarting after a short recovery period during which there is specifically no spark (Critical Diagnosis Factor). Recovery involves waiting for the Mitsubishi power
- transistor encapsulated within the ignitor module to undergo thermal reset. The failure mode becomes increasingly frequent as the semiconductor suffers irreversible degradation with each incident. No ECU code is set during this period, thus the symptomatic behaviour and affected years should be the focus of any investigation. UK owners can now obtain discount parts from Ford UK, Coil: 3974598 at £33.15 & Stator: 3974599 at £33.15 (both exclude VAT @ 17.5%). The TSB is known to all Ford main dealers.
5-Jan-95 V6s
- – Cars built after 5th January 1995 date have essentially no problems. They are unaffected by carbon-knock (decarbon service) or rear-calliper handbrake seizure (Sumitomo redesigned, UK owners ex-gratia replacement) or ignitor problem (as above). In commong with MX5/Miata the very deep spark-plug-well design requires new ignition leads every 5yrs. Replacement ignition leads range from discounted Mazda leads from Roebuck Mazda USA, to NGK/Bosch at £87/63$US and Magnecor at £90/140$US).